Global navigation satellite systems (GNSS), such as the American NAVSTAR GPS and Russian GLONASS, are known. The NAVSTAR GPS developed by the U.S. Defense Department is a satellite-based radio navigation system which transmits information from which extremely accurate navigational calculations can be made in three-dimensional space anywhere on or near the Earth. Three-dimensional velocity can be determined with equal precision. The GPS uses 24 satellites dispersed in six, inclined, 12 hour circular orbits chosen to insure continuous 24 hour coverage world-wide. Each satellite uses extremely accurate cesium and rubidium vapor atomic clocks for generating a time base. Each satellite is provided with clock correction and orbit information by Earth-based monitoring stations.
Each satellite transmits a pair of L-band signals. The pair of signals includes an L1 signal at a frequency of 1575.42 MHZ and an L2 signal at a frequency of 1227.6 MHZ. The L1 and L2 signals are bi-phase modulated by pseudo-random noise (PRN) codes and an information signal (i.e., navigational data) encoded at 50 Hz. The PRN codes facilitate multiple access through the use of a different PRN code by each satellite.
Upon detecting and synchronizing with a PRN coded signal, a receiver decodes the signal to recover the navigational data, including emphemeris data. The emphemeris data is used in conjunction with a set of Kepler equations to precisely determine the location of each satellite. The receiver measures a phase difference (e.g., time of arrival) of signals from at least four satellites. The time differences are then used to solve a matrix of four equations to provide a space and time solution. The result is a precise determination of location of the receiver in three-dimensional space.
The velocity of the receiver may be determined by a precise measurement of the L1 and L2 frequencies. The measured frequencies are used to determine Doppler frequency shifts for each satellite. The measured differences are used to solve another set of equations to determine a velocity of the receiver relative to the Earth based upon the detected phase shift of the received signals.
Integrated GPS/INS systems require an initialization of system attitude (i.e., orientation relative to the Earth) in terms of roll, pitch and azimuth. The INS portion of a GPS/INS system typically performs a self-determination of roll and pitch through internally sensed acceleration. In high quality INS systems, azimuth is determined by sensing the horizontal component of the Earth's rotation rate, which provides an indication of North. This technique is not available to less sophisticated (less accurate) GPS/INS systems, often requiring that an external source provide azimuth initialization (e.g., an operator periodically orienting the INS system towards a northerly direction using a compass and activating an azimuth initialization button). Accordingly, a need exists for a method of determining azimuth that does not depend upon high quality accelerometers or gyroscopes.